Since list is an interface, one can’t directly instantiate it. However, one can create objects of those classes which have implemented this interface and instantiate them.
Few classes which have implemented the List interface are Stack, ArrayList, LinkedList, Vector etc.
Syntax:
List list=new ArrayList[]; List llist=new LinkedList[]; List stack=new Stack[];Examples:
|
import java.util.*; import java.util.function.Supplier; public class GFG { public static void main[String args[]] { List list = new ArrayList[]; list.add[1]; list.add[3]; System.out.println["ArrayList : " + list.toString[]]; List llist = new LinkedList[]; llist.add[2]; llist.add[4]; System.out.println["LinkedList : " + llist.toString[]]; List stack = new Stack[]; stack.add[3]; stack.add[1]; System.out.println["Stack : " + stack.toString[]]; } } |
Output: ArrayList : [1, 3] LinkedList : [2, 4] Stack : [3, 1]
Double Brace Initialization can also be used to do the above work.
Syntax:
List list=new ArrayList[]{{ add[1]; add[2]; add[3]; }};Examples:
|
import java.util.*; public class GFG { public static void main[String args[]] { List list = new ArrayList[] {{ add[1]; add[3]; } }; System.out.println["ArrayList : " + list.toString[]]; List llist = new LinkedList[] {{ add[2]; add[4]; } }; System.out.println["LinkedList : " + llist.toString[]]; List stack = new Stack[] {{ add[3]; add[1]; } }; System.out.println["Stack : " + stack.toString[]]; } } |
Output: ArrayList : [1, 3] LinkedList : [2, 4] Stack : [3, 1]
-
Creating Immutable List
Arrays.asList[] creates an immutable list from an array. Hence it can be used to instantiate a list with an array.
Syntax:
List list=Arrays.asList[1, 2, 3];Examples:
import java.util.Arrays;
import java.util.List;
public class GFG {
public static void main[String args[]]
{
List list = Arrays.asList[1, 2, 3];
System.out.println["List : " + list.toString[]];
}
}
-
Creating Mutable List
Syntax:
List list=new ArrayList[Arrays.asList[1, 2, 3]];Examples:
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class GFG {
public static void main[String args[]]
{
List list = new ArrayList[
Arrays.asList[1, 2, 3]];
System.out.println["List : " + list.toString[]];
list.add[5];
System.out.println["Modified list : " + list.toString[]];
}
}
Output: List : [1, 2, 3] Modified list : [1, 2, 3, 5]
There are various methods in Collections class that can be used to instantiate a list. They are:
-
Collections class has a static method addAll[] which can be used to initialize a list. Collections.addAll[] take in any number of elements after it is specified with the Collection in which the elements are to be inserted.
Syntax:
List list = Collections.EMPTY_LIST; Collections.addAll[list = new ArrayList[], 1, 2, 3, 4];Examples:
import java.util.*;
public class GFG {
public static void main[String args[]]
{
List list = new ArrayList[];
Collections.addAll[list, 1, 2, 3, 4];
System.out.println["List : " + list.toString[]];
}
}
Output: List : [1, 2, 3, 4]
-
Collections.unmodifiableList[] returns a list which can’t be altered i.e. it can neither add or delete an element. Any attempt to modify the list will result in an UnsupportedOperationExample.
Syntax:
List list = Collections .unmodifiableList[Arrays.asList[1, 2, 3]];Example 1:
import java.util.*;
public class GFG {
public static void main[String args[]]
{
List list = Collections.unmodifiableList[
Arrays.asList[1, 2, 3]];
System.out.println["List : " + list.toString[]];
}
}
Example 2:
import java.util.*;
public class GFG {
public static void main[String args[]]
{
try {
List list = Collections.unmodifiableList[
Arrays.asList[1, 2, 3]];
System.out.println["List : " + list.toString[]];
System.out.println["Trying to modify the list"];
list.set[0, list.get[0]];
}
catch [Exception e] {
System.out.println["Exception : " + e];
}
}
}
Output: List : [1, 2, 3] Trying to modify the list Exception : java.lang.UnsupportedOperationException
-
Collections.singletonList[] returns an immutable list consisting of one element only.
Syntax:
List list = Collections.singletonList[2];Example 1:
import java.util.*;
public class GFG {
public static void main[String args[]]
{
List list = Collections.singletonList[2];
System.out.println["List : " + list.toString[]];
}
}
With the introduction of Stream and functional programming in Java 8, now one can construct any stream of objects and then collect them as a list.
Syntax:
1. List list = Stream.of[1, 2, 3] .collect[Collectors.toList[]]; 2. List list = Stream.of[1, 2, 3] .collect[Collectors.toCollection[ArrayList::new]]; 3. List list = Stream.of[1, 2, 3, 4] .collect[Collectors.collectingAndThen[Collectors.toList[], Collections::unmodifiableList]];Examples:
|
import java.util.*; import java.util.stream.Collectors; import java.util.stream.Stream; public class GFG { public static void main[String args[]] { List list1 = Stream.of[1, 2, 3] .collect[Collectors.toList[]]; System.out.println["List using Syntax 1: " + list1.toString[]]; List list2 = Stream .of[3, 2, 1] .collect[ Collectors .toCollection[ArrayList::new]]; System.out.println["List using Syntax 2: " + list2.toString[]]; List list3 = Stream .of[1, 2, 3, 4] .collect[ Collectors .collectingAndThen[ Collectors.toList[], Collections::unmodifiableList]]; System.out.println["List using Syntax 3: " + list3.toString[]]; } } |
Output: List using Syntax 1: [1, 2, 3] List using Syntax 2: [3, 2, 1] List using Syntax 3: [1, 2, 3, 4]
Java 9 introduced List.of[] method which takes in any number of arguments and constructs a compact and unmodifiable list out of them.
Syntax:
List unmodifiableList = List.of[1, 2, 3];Examples:
|
import java.util.List; public class GFG { public static void main[String args[]] { List unmodifiableList = List.of[1, 2, 3]; System.out.println["List : " + unmodifiableList.toString[]]; } } |
OUTPUT:
[1, 2, 3]